Effects of the North Atlantic Oscillation on precipitation-type frequency and distribution in the eastern United States

The North Atlantic Oscillation (NAO) is the primary mode of atmospheric variability over the Atlantic Ocean and plays an important role in climate variability over eastern North America. The frequency of extreme climate events and the associated social and economic impacts is also tied to the strength and phase of the NAO. In this study, seasonal phases of the NAO are compared to changes in the frequency and distribution of winter season (December-March) precipitation-type observations for the years 1961-2001 in the eastern U.S. Statistically significant increases in the frequency of rain observations across the study region are associated with positive NAO phases. Additionally, significant increases in the occurrence of snow are confined to the northern portion of the eastern U.S. during positive NAO phases. Connections between the phase of the NAO and the prevailing synoptic-scale circulation, at least partially, explain the inter-seasonal distribution of precipitation types across the eastern U.S. Using an intra-seasonal or intra-monthly NAO index may reveal a more robust relationship between snowfall observations across the eastern U.S. and surface pressure variability over the North Atlantic.